Repeater for tdma mobile radio

ABSTRACT

A repeater for enhancing performance of a TDMA mobile radio system in poor signal areas has a bank of frequency agile, single channel amplifiers (13), and controller (14). The controller (14) scans the channel of the band and, upon identifying a channel carrying traffic, enables one of the amplifier units (13) to operate on that channel. Each of the amplifier units (13) includes a time slot activity detector (35) for detecting which time slot of a channel is active. The output of the activity detector (35) is passed to a logic control module (36), which controls the amplification of an active time slot via a time slot gate and power unit (38).

BACKGROUND

I. Field of the Invention

This invention relates to a mobile radio system, and in particular to aradio repeater (cell enhancer) for a digitaltime-division-multiple-access (TDMA) mobile radio system.

II. Prior Art and other Considerations

In a mobile radio system, there often exist areas of poor coveragewithin the service area of a base station, where poor signal receptionby the base station from a mobile (and vice versa) results inunacceptable voice quality, or even the total loss of a call. Onesolution to this problem is the provision of a wideband repeater orre-radiator. This however suffers from certain disadvantages, viz (i) itwould re-radiate all signals within the frequency rangecovered-including unwanted signals, for example from other radio systemswhich may be using the band; (ii) intermodulation and wideband noise;and (iii) the power level of each channel within the band would beamplified by an equal amount regardless of the actual signal strength.

A radio repeater is known (see the specification of our co-pending EPpatent application no. 0274857) which utilizes a plurality offrequency-agile single-channel amplifier units for amplifying onlytraffic-carrying channels. This repeater is an analoguefrequency-division-multiple-access (FDMA) system, and so is not suitablefor digital systems such as the proposed pan-European GSM (GroupeSpeciale Mobile) mobile radio system.

SUMMARY

The present invention provides a radio repeater for a frequency bandhaving a plurality of RF channels for carrying a plurality of digitalTDMA channels each having a sequence of time slots, comprising: aplurality of single RF-channel amplifier units; and control means forscanning the RF channels to identify an active RF channel and forassigning one of the amplifier units to an identified active channel,characterised in that each of the amplifier units has means foridentifying an active time-slot of a TDMA channel, within an active RFchannel, and for amplifying digital signals in that time slot.

Advantageously, each amplifier unit has, for duplex operation, forwardand reverse amplifying paths, and the number of amplifier units issmaller than the number of the RF channels, the amplifier units being ofselectable frequency, and the control means being arranged, in use, toselect the frequency of each unit for assignment to an active channel.

In one preferred embodiment, each amplifier unit has, in each amplifyingpath, a first frequency changer for converting input signals down to anintermediate frequency, a band-pass filter, and a second frequencychanger for converting the filtered signals up to the originalfrequency, the filter frequency being fixed, and the channel assignmentof the unit being controllable by controlling the frequency changerlocal oscillator frequency. Conveniently, the intermediate frequenciesfor the forward and reverse paths of an amplifier unit differ by anamount corresponding to the transmit-receive frequency spacing of thespeech channels, whereby a single local oscillator may be used for theunit. Preferably, the control means is common to all the amplifierunits.

In another preferred embodiment, each amplifier unit has, in eachamplifying path, a frequency changer for converting input signals to anintermediate frequency, a band-pass filter, a frequency discriminatorfor demodulating the digital signal, a regenerator for the recovereddigital signal, a modulator driven by the regenerated digital signal andoperating at the original frequency, the channel assignment of the unitbeing controllable by controlling the local oscillator frequenciessupplied by the frequency changer and the modulator. The output of themodulator is amplified to produce the output of the amplifier unit.

Alternatively, the frequency changer may comprise two mixers bothsupplied with the input signal, the local oscillator for each mixerbeing at the same frequency but with a relative phase displacement of 90degrees, with the local oscillator frequency being nominally the same asthe signal frequency. The resulting two signals from the mixers areprocessed at baseband to derive a regenerated replica of the digitalmodulation, preferably by digital signal processing techniques. Theprocessing includes low-pass filtering. The regeneration processproduces two signals which are applied to two modulators driven by thesame local oscillator as the frequency changer, and with a relativephase displacement of 90 degrees. The outputs of the two modulators arecombined and amplified to produce the output of the amplifier unit.

Advantageously, the time slot identification means comprises a time slotactivity detector for detecting which time slot of a channel is active,a control unit which receives the output of the time slot activitydetector, and a time slot gate and power control unit which iscontrolled by the control unit so as to amplify only an active timeslot. In this case, each amplifier unit may further comprise a clockrecovery, synchronisation and time slot detector unit for deriving asystem clock, synchronisation pulses and time slot strobes and applyingthese to the control unit.

Preferably, the control unit of each amplifier unit includes means forcontrolling the operating frequency of the enabled amplifier unit tofollow a predetermined frequency hopping sequence, thereby allowing therepeater to be used in radio systems which employ frequency hopping.Conveniently, the frequency control means of a given amplifier unit iseffective to control the first and second frequency changers ormodulators of that amplifier unit.

BRIEF DESCRIPTION OF THE DRAWINGS

A radio repeater constructed in accordance with the invention, will nowbe described, by way of example, with reference to the accompanyingdrawings, in which:

FIG. 1 shows schematically a mobile radio area;

FIG. 2 is a block diagram of the repeater;

FIG. 3 is a block diagram of one amplifier unit of the repeater of FIG.2;

FIG. 4 is a block diagram of the RF module of the repeater of FIG. 2;

FIG. 5 is a block diagram of an alternative embodiment of one amplifierunit of the repeater of FIG. 2; and

FIG. 6 is a block diagram of another alternative embodiment of oneamplifier unit of the repeater of FIG. 2.

DETAILED DESCRIPTION OF THE DRAWINGS

Referring to the drawings FIG. 1 illustrates schematically a mobileradio system, in which a base station 1 serves an area 2--perhaps one"cell" of a cellular radio system for two-way speech communication witha number of mobile units 3. It is assumed that, within a given frequencyband, duplex radio channels, namely a control or signalling channel anda number of speech channels, are allocated for communication between thebase station 1 and the mobile units 3 within the area 2. Owing (forexample) to the nature of the terrain, there exists within the area 2 apoor coverage area 4. The quality of communication between the basestation 1 and the mobile units 3 within this area 4 is unsatisfactory,and a fixed repeater 5 is provided at a location such that it canachieve satisfactory radio communication both with the base station andwith the mobile units within the poor coverage area.

FIG. 2 illustrates the repeater 5 which has, in outline, an antenna 10for communication with the base station 1, and an antenna 11 forcommunication with the mobile units 3 within the poor coverage area 4.Between the two antennas 10 and 11 are connected, via a radio frequency(RF) module 12, a number (three only of which are shown) of duplex,single-channel amplifier modules 13. The number of modules 13 is lessthan the number of channels allocated for the radio area 2, and acontrol unit 14 is provided for channel allocation. The control unit 14scans the allocated channels to identify those which are in use, andswitches the amplifier units 13 to the appropriate channels.

The scanning function may be performed by one or more of the amplifierunits under the control of the control unit (14).

An amplifier module 13 is shown in FIG. 3. In order to achieve theselectivity required for channel selection, incoming signals areconverted down to an intermediate frequency (IF), filtered andup-converted. Though not in principle essential, the frequency ofre-radiation is the same as that of the received signals, to avoid thenecessity of requiring the mobile unit 3 (or the base station 1) tochange frequency.

In FIG. 3, the signal received at an input 20 from the base station 1 isdown-converted in a mixer 21 to an intermediate frequency of (forexample) 45 MHz, and the desired channel is filtered in a band-passfilter 23 and amplified by amplifier 24. It is then up-converted to theoriginal frequency (by a mixer 25), amplified in a power amplifier 26,and supplied to an output 27 for retransmission to the poor coveragearea 4. This base-to-mobile transmission direction is known as adownlink. The mobile-to-base transmission direction (the uplink) istreated similarly by units 20' to 27', except that (assuming atransmit-receive frequency spacing of 45 MHz) a 90 MHz I.F. is chosen,so that a single output from a local oscillator 28 can be used to supplyall four mixers 21, 25, 21', 25'. The I.F. filters 23, 23' are fixedfrequency, and channel selection is effected by controlling thefrequency of the local oscillator 28. The oscillator 28 is aconventional frequency synthesiser capable of being switched, via acontrol bus 15 (see FIG. 2) from the control unit 14, to frequenciessuch that the I.F. filters 23, 23' select any of the allocated channels.Similarly, the control unit 14, via the bus 15, can control theamplifier 13, turning the individual time slots on or off, and settingthe output signal level to a value derived by the control unit 14 fromthe incoming signal.

The RF module 12 is shown in FIG. 4. The "base" antenna 10 is connectedvia a duplexer 30 to (a) a receive preamplifier 31 and a power splitter32 which feeds the module inputs 20 (and, if required, the control unit14), and (b) a passive power combiner 33 which receives the moduleoutputs 27'. Similar components 30' to 33' interface the "mobiles"antenna 11 to the module inputs 20' and outputs 27. In an alternativeembodiment (not shown), separate antennas 10 and 11 could be used foreach amplifier module 13.

The architecture of control unit 14 can be conventional and will not,therefore, be described in detail.

In the present embodiment of the invention, the function of the controlunit 14 is to determine to which frequencies the channel modules 13should be switched. The control unit is programmed with a set of channelfrequencies on which the repeater may operate. When a signal in atime-slot on any of these channels meets certain predeterminedconditions, the channel is judged to be in use. If a channel module isavailable, it will be switched on to amplify that time-slot in thatchannel. The conditions for determining the in-use state may includesignal strength, signal to noise ratio, and quality of the digitalmodulation.

Programming of the control unit may be by manual means, for exampleswitches or keypad attached to the unit, or remotely via an appropriatecommunications interface, or by resident internal program.

Where the repeater is equipped with fewer channel modules than thenumber of frequency channels on which it is programmed to operate, thenduring periods in which one or more channel modules are not occupied inamplifying, they may be used to scan the remaining relevant channels foractivity. Alternatively, a dedicated scanning receiver module may beused for this purpose.

In the cellular system, the channels to scan will consist of one controland signalling channel and a number of speech channels (the control andsignalling channel is used for registration of mobile transmitters ontothe cellular system, for setting up telephony calls, and for othercontrol purposes such as initiating frequency hopping sequences).

For a TDMA mobile radio system, each mobile unit 3 is allocated a giventime slot in the time domain by the base station 1. This time slotallocation is carried out during the call set-up procedure. Typically,the time domain has eight time slots, which effectively means that therepeater 5 can handle eight times as many mobile units 3 as there arechannels.

As shown in FIG. 3, the antennas 10 and 11 are connected to each of theduplex amplifier units 13 via duplexers 30 and 30'. The duplexers 30 and30' are common to all the amplifier units 13, and form part of the RFmodule 12. In an alternative embodiment which utilises separate antennas10 and 11 for each amplifier 13, time switches could be used instead ofthe duplexers 30 and 30'.

Once an amplifier unit is assigned to an operating channel, the receivedsignals on the downlink are intercepted and amplifier by amplifier 34and passed to the demodulator and level detector 35, which also derivestime-slot timing information. A similar function on the uplink isperformed by amplifier 34' and demodulator and level detector 35'.

The control logic module 36 uses the signal level and time-slot timinginformation from 35 and 35' to control the transmitted power levels inthe relevant time-slots of downlink and uplink, via the time-slot gateand power controllers 38 and 38', so as to amplify only signals in therelevant time-slots.

It should be noted that the time-slots proper to a given mobile have arelative time displacement between uplink and downlink of severaltime-slots.

FIG. 5 illustrates schematically an alternative embodiment of anamplifier module 13, in which the received signal at input 20 isdown-converted in a mixer 21 to an intermediate frequency of (forexample) 45 MHz, and the desired channel is filtered by a band-passfilter 23 and amplified by amplifier 24. The signal is then demodulatedby demodulator 40 and the digital modulating signal is regenerated inregenerator 41, which also derives signal level and time-slot timinginformation for the control logic module 36. The recovered digitalsignal is applied to modulator 25 to produce a replica of the oringinalsingal, which is amplifier in amplifier 26 and supplied to an output 27for retransmission. The othe direction of transission is treatedsimilarly in units 21' to 245', 40', 41', 25' and 26'.

FIG. 6 illustrates yet another alternative embodiment of an amplifiermodule 13, in which the received signal at input 20 is applied to twomixers 50 and 51, the local oscillator for each mixer being at thenpominal center frequency of the channel to be amplified. The localoscillator supplies to the two mixers are displaced in phase by 90degrees in order to operate the mixers in phase quadrature. Theresulting mixer outputs represent the quadrature modulation componentsof the received singal, and are applied to low-pass filiters 53 and 54which attenuate any unwanted high frequency signals. The outputs fromfilters 53 and 54 are applied to a signal-processing unit 55 whichrecovers the digital modulation and derives time-slot timimginformation, and also detects the level of the received signal. Unit 55may also include means for equalisation, error detection and errorcorrection. Mixers 56 ands 57 are supplied with a local oscillator atthe channel frequency via a 90 degree phase shift unit 58, and are alsosupplied with appropriate outputs from signal processing unit 55, so asto recreate a replica of the received signal. The other direction oftransmission is handled in a similar manner by units 50' to 57' and 26'.

As the GSM mobile radio system has a requirement for frequency hopping,the amplifier units 13 are each provided with means for implementing ahopping sequence. The arrangement is such that the system operator candecide whether or not frequency hopping is to be used. Where it is to beused, the control unit 14 instructed accordingly by the control andsignalling channel, and this unit sends appropriate control instructionsto the control logic modules 36 of the amplifier units 13. The modules36 then control their synthesisers 2B, which drive the mixers 21, 21',25 and 25', to implement the predetermined hopping sequence.

This hopping sequence is controlled by software associated with the basestation 1. Each of the mobile units 3 is provided with means forcontrolling the pattern of frequency hopping for that mobile unit; onceinstructions are received over the control channel from the base station1.

We claim:
 1. A radio repeater for a frequency band having a plurality of RF channels for carrying a plurality of digital TDMA channels each having a sequence of time slots, comprising:a plurality of single RF-channel amplifier units; control means for scanning the RF channels to identify an active RF channel and for assigning one of the amplifier units to an identified active channel; wherein each of the amplifier units has means for identifying an active time-slot of a TDMA channel, within an active RF channel, and for amplifying digital signals in that time slot.
 2. A repreated as claimed in claim 1, wherein each amplifier unit has, for duplex operation, forward and reverse amplifying paths.
 3. A repeater as claimed in claim 1 wherein the number of amplifier units is smaller than the number of said RF channels, the amplifier units being of selectable frequency, and the control means being arranged, in use, to select the frequency of each unit for assignment to a traffic-carrying speech channel.
 4. A repeater as claimed in claim 3, wherein each amplifier unit has, in each amplifying path, a first frequency changer for converting input signals down to an intermediate frequency, a band-pass filter, and a second frequency changer for converting the filtered signals up to the original frequency, the filter frequency being fixed, and the channel assignment of the unit being controllable by controlling the frequency changer local oscillator frequency.
 5. A repeater as claimed in claim 3, wherein each amplifier unit has, in each amplifying path, means for demodulating and regenerating the digital modulating signal and a modulator for recreating a modulated singal at the received frequency using said digital signal.
 6. A repeater as claimed in claim 5, wherein digital signal processing techniques are used for any or all of the functions of filtering, demodulation and regeneration, and optically for equalisation of distortion of the received signal.
 7. A repeater as claimed in claim 4, wherein the intermediate frequencies for the forward and reverse paths of an amplifier unit differ by an amount corresponding to transmit-receive frequency spacing of the speech channels, whereby a single local oscillator may be used for the unit.
 8. A repeater as claimed in claim 1, wherein the control means is common to all the amplifier units.
 9. A repeater as claimed in claim 1, wherein the time slot identification means comprises a time slot activity detector for detecting which time slot of a channel is active, a control unit which receives the output of the time slot activity detector, and a time slot gate and power control unit which is controlled by the control unit so as to amplify only an active time slot.
 10. A repeater as claimed in claim 9, wherein each amplifier unit further comprises a clock recovery, synchronisation and time slot detector unit for deriving a system clock synchronisation pluses and time slot strobes and applying these to the control unit.
 11. A repeater as claimed in the claim 9, wherein the control unit of each amplifier unit includes means for controlling the operating frequency of the enabled amplifier unit to follow a predetermined frequency hopping sequence, thereby allowing the repeater to be used in radio systems which employ frequency hopping
 12. A repeater as claimed in claim 11, wherein the frequency control means of a given amplifier unit is effective to control the first and second frequency changers to that amplifer unit.
 13. A radio repeater for a frequency band having a plurality of RF channels for carrying a plurality of digital TDMA channels each having a sequence of time slots, comprising:a plurality of single RF-channel amplifier units; control means for scanning the RF channels to identify an acitve RF channel and for assigning one if the amplifier units to an indentified active channel; wherein each of the amplifier units has means for identifying an active time-slot of a TDMA channel, within an active RF channel, and for amplifying digital signals in that time slot, the time-slot identifying means including:a time-slot activity detector for detecting which time slot of a channel is active; means responsive to the time slot activity detector for amplifying only an active time-slot.
 14. A method for operating a radio repeater for a frequency band having a plurality of RF channels for carrying a plurality of digital TDMA channels each having a sequence of time slots, the method comprising:scanning the RF channels to identify an active RF channel; assigning one plurality of single RF-channel amplifier units to an identified active channel; and, using the assigned one of the plurality of single RF-channel amplifier units to identify an active time-slot of a TDMA channel, within an active RF channel, and to amplify digital signals in that time-slot. 